The present invention relates to an electrophotographic copying machine of a type having a plurality of magnification modes and, more particularly, to an adjustment means for controlling the correlation of members constituting an optical system to suit a particular magnification mode.
Up to the present, so far as an optical system capable of effecting various magnifying powers in any type of copying machine is concerned, there have been already proposed a number of arrangements and improvements thereof. According to such conventional arrangements, the power of magnification is in general varied by shifting the mirrors so as to readjust the conjugate distance (the distance between an original image carrying surface and a photosensitive or photoreceptor surface), with the lens assembly being also moved so as to change the magnifications. However, almost all the conventional systems provide arrangements which only achieve a reduction in the magnifying power. From a constructional point of view, according to the conventional system, the control of movement of the mirrors is ordinarily arranged to be performed by a cam mechanism provided in the system. Such being the case, it is quite unreasonable to except that precise control in positioning of the mirrors can be effected in a predetermined manner only by increasing the precision of the cam mechanism, without taking into account, for example, the surface condition of the mirrors, errors in focal length occurring in the manufacture of the lens assembly or the like. Thereby, as a result, a focusing adjustment per se can not be omitted for every variation of the magnification mode.
Referring now to FIG. 1, there is shown an explanatory diagram particularly showing the relationship between the conjugate distance mentioned above and the property specific to the lens in connection with the variation of the magnification mode. More specifically, there exist the following two relationships between the relative amount of variation of the conjugate distance .DELTA.L and the relative amount of movement l of the lens assembly or the lens, relative to a full size reproduction mode. ##EQU1## .beta.: the magnification f: the focal length of the lens assembly
For example, according to the relationships (1) and (2), when a case wherein the focal length f and the magnification .beta..sub.1 are 280 mm and 1.414, respectively and another case where f=280 mm and .beta..sub.2 =0.707 are compared, the relative amounts of variation of the conjugate distance are the same and have the same value of .DELTA.L.apprxeq.34 mm. However, respective amounts of movement of the lens assembly are different, with l.sub.1 .apprxeq.82 mm for the former case and l.sub.2 .apprxeq.116 mm for the latter case.
In general, in view of the manufacturing error involved as described above, an error of approximately one percent is ordinarily inherent in the focal length of the lens assembly and therefore, the focusing error for the lens assembly having a f-value of 280 mm correspondingly has an approximate value of .+-.3 mm. In association with the error as described above, an error of approximate .+-.12 mm is in turn caused in the conjugate distance. Therefore, when the apparatus is converted from the full size reproduction mode of a reduced size mode having the magnification of 0.707, the error of the relative amount of variation of the conjugate distance .DELTA.L is correspondingly caused to be approximately .+-.0.4 mm. As a matter of fact, in addition to the error described above, when such effects as are caused, for example, by surface conditions of the mirrors, mechanical precision of the cam mechanism and the like are taken into consideration, it is clear that precise control in positioning of the mirrors can be hardly performed simply by the conventional cam mechanism. Therefore, according to the further improved conventional devices, the focusing adjustment is performed by the precise adjustment of means for determining the locations of the mirrors, with the lens assembly per se being disposed at a predetermined location by a stationary stop.
As far as an optical system arranged to achieve only one reduced size reproduction mode is concerned, the arrangement of the optical system described above achieves a good result without causing any operating defects. However, with respect to an optical system, wherein a plurality of magnification modes are to be provided, such conventional means as described above will not achieve a satisfactory result. Moreover, in an optical system including at least one paired reduced and enlarged size reproduction modes each having the same or approximately the same conjugate distance, although the conjugate distances are the same or approximately the same as shown in FIG. 1, the optical system of the prior art as described above is still not adequate to permit omitting the means for focusing adjustment, i.e., the means for adjustment of the locations of the mirrors, for every different magnification mode. More specifically, when the conventional prior art apparatus, wherein the mirrors are specifically moved by the movement of a single cam, with the lens assembly at each predetermined position being positioned by a stationary stop for each mode, is used for a paired optical system as described above, the locations of the mirrors effected in association with the reduced size reproduction mode (or alternatively, the enlarged size reproduction mode) will be shifted from their fixed locations by an adjustment of the mirrors effected in association with the enlarged reproduction mode (or alternatively, the reduced size reproduction mode), with the result that the conventional apparatus as described above is substantially not applicable.